In order to decrease capacity of a low voltage battery for a vehicle is used in an existing hybrid vehicle or electric vehicle, a buck converter is used. The buck converter converts a high voltage of a high voltage battery for a hybrid vehicle or an electric vehicle into a low voltage. The converted low voltage is charged in a low voltage battery of a vehicle, and is used as a power source of an electric field load.
An output controller monitors an output voltage of the buck converter, compares the monitored output voltage with a reference voltage, and then generates output in proportion to a size of a vehicle.
In the related art, the converter is controlled by controlling an On-time of a pulse width modulation (PWM) signal only with the output generated by the output controller and a chopping wave of a fixed frequency. In this case, there exists a problem in that a response speed against a rapid change of load current is slow.